1. Field of the Invention
This invention relates generally to apparatus for reproducing video and audio signals, such as may constitute a television signal, that have been recorded on a magnetic medium and, more particularly, is directed to apparatus to compensate for drop-outs occurring at random that have adverse effects on the reproduced signals.
2. Description of the Prior Art
In the video tape recorders, according to the prior art, for recording a color television signal on a magnetic tape, the chrominance and luminance signal components that constitute the color video signal are separated, and the carrier frequency of the chrominance signal component is converted to a low value relative to the higher carrier frequency that the luminance component frequency modulates, whereupon the frequency-converted chrominance signal component and the high side band of the frequency-modulated luminance signal component are mixed or combined to form a composite color video signal that is recorded on a magnetic tape in successive, parallel record tracks extending obliquely in respect to the longitudinal or running direction of the magnetic tape. In such existing system for recording a color television signal, the audio signal is recorded on the magnetic tape in separate record tracks that extend in the longitudinal direction, that is, the running direction of the magnetic tape.
In the above described recording system of the prior art, the oblique or slanted tracks that are skewed relative to the tape, in which the composite color video signal made up of the frequency-converted chrominance signal component and the frequency-modulated luminance signal component is recorded, are formed by first and second substantially diametrically opposed rotary magnetic heads both supplied with the composite color video signal. The magnetic heads alternately scan the magnetic tape along a path at an angle to the longitudinal direction in which the tape is transported. In order to increase the recording density of the color video signal on the tape, and thereby increase the duration of the recording, it has been known to restrict the speed at which the magnetic tape is transported so that the successive slanted tracks scanned by the rotary magnetic heads will be more closely or immediately adjacent each other, that is, so that the spaces or so-called guard bands between the adjacent slanted tracks will be eliminated. Nevertheless, in such case, the problem of "cross talk" arises in the reproduction or playback mode of the apparatus. In other words, during reproducing or playback of these closely arranged tracks, a transducer or head scanning one of the slanted tracks for reproducing the composite color video signal recorded therein will also pick up or reproduce signals from the next adjacent tracks, such signals being known as cross talk.
This problem of cross talk has been substantially solved, at least in respect to the relatively high frequency frequency-modulated luminance signal component of the recorded composite color video signal, by providing the first and second rotary magnetic heads with different azimuth angles so that the composite color video signal will be recorded in each slanted track by a magnetic head having an azimuth angle different from the azimuth angle of the head with which the composite color video signal is recorded in the next adjacent track. Subsequently during reproducing or playback, each slanted track is scanned by the rotary magnetic head having the corresponding azimuth angle, with the result that a substantial and beneficial azimuth loss is experienced in the relatively high frequency components of the cross talk derived from the adjacent tracks. Thus, the cross talk in respect to the frequency-modulated luminance signal component is substantially suppressed.
Nevertheless, the azimuth loss effect is rather poor in respect to the low frequency band of the cross talk, that is, in respect to the frequency-converted chrominance signal component, so that other measures need be taken to eliminate or minimize the low-frequency component of the cross talk. As disclosed in detail in U.S. Pat. No. 4,007,482, issued Feb. 8, 1977, and having a common assignee herewith, cross talk in respect to the frequency-converted chrominance signal situated in a relatively low-frequency band is substantially eliminated by recording the chrominance signal component with different first and second carriers in the adjacent tracks, respectively. These first and second carriers, which have been modulated by the chrominance signal component and recorded in adjacent tracks, respectively, may be distinguished from each other by their respective frequency and/or polarity characteristics so that, upon reproduction of the signal recorded in a particular track, the low frequency component of the cross talk from the tracks next to it can be suppressed or eliminated by reason of the different frequency, and/or polarity, or phase characteristics of the respective carriers.
More specifically, as disclosed in the above-identified U.S. patent, the chrominance signal component of the color video signal to be recorded may be frequency-converted so as to produce selectively first and second frequency converted signals which, when considered instantaneously, have the same carrier frequency but which differ from each other in their phase or polarity characteristics. In this case, each of the line areas of one track may have recorded therein a frequency-converted chrominance signal component with a carrier of constant polarity, while in the next adjacent tracks the carrier of the frequency-converted chrominance signal component recorded therein reverses its polarity for successive line intervals. Such pattern of recording ensures that, during playback or reproduction, cross-talk effects can be minimized or eliminated.
When high density recording of the color video signal in successive slant tracks on the tape is effected as described above, the rotational speed of the rotary magnetic heads is relied upon to provide the desired relatively high speed in respect to the magnetic tape for ensuring high quality recording of the color video signal in the slanted or skewed tracks. Nevertheless, in achieving the high density recording of the color video signal the transport speed of the magnetic tape is necessarily quite low, thus, the relative velocity between the magnetic tape and the fixed heads, which record the audio signals in the respective audio tracks, is also quite low, with the result that the quality of the audio recording is adversely affected.
It has been proposed that the audio signals be frequency modulated and then mixed with the composite color video signal to provide a mixed or combined signal supplied to the rotary magnetic heads for recording the combined signal in the slanted tracks. It has also been proposed that the video and audio signal recording apparatus comprise frequency modulating means for modulating first and second carriers by a first audio signal to be recorded, for example, by a stereophonic left signal, thereby providing first and second frequency-modulated audio signals, respectively, and for modulating third and fourth carriers by a second audio signal to be recorded, by a stereophonic right signal, thereby providing third and fourth frequency-modulated audio signals, respectively. These first, second, third, and fourth carriers would all have different frequencies. The proposed apparatus also includes a first mixing means for mixing, with the video signal, two of the frequency-modulated FM audio signals that represent the first and second audio signals to be recorded, respectively, and thereby providing a first mixed audio and video signal, and also second mixing means for mixing the other two of the frequency-modulated audio signals with the video signal, thereby providing a second mixed audio and video signal. This system further teaches that the first and second magnetic heads have different azimuth angles and receive the first and second mixed audio and video signals, respectively, for recording the first and second mixed signals in respective record tracks that are adjacent each other on the magnetic tape.
The respective frequencies of the first, second, third, and fourth carriers of the frequency modulated audio signals, as described above, can have successively increasing values, wherein the first frequency-modulated audio signals mixed with the video signal in the first mixing means for providing the first mixed audio and video signal are the first and third frequency-modulated audio signals, and the other two frequency-modulated audio signals mixed with the video signal to provide the second mixed audio and video signal are the second and fourth frequency-modulated audio signals.
When reproducing video and audio signals recorded as first and second mixed signals, as described above, with different azimuth angles in adjacent parallel tracks on a magnetic record medium, the adjacent tracks are scanned by first and second magnetic heads having corresponding azimuth angles so as to reproduce alternately the first and second mixed signals. The reproduced signals are then separated into the respective frequency-modulated audio signals from the reproduced first and second mixed signals, and the frequency-modulated audio signals that have been separated from the first and second mixed signals are frequency demodulated, thereby to obtain respective alternately reproduced demodulated portions of the first and second audio signals. These alternately reproduced signals are then sequentially combined to provide substantially continuous demodulated first and second audio signals.
Thus, it is known to have high density recording of mixed audio and video signals in a fashion such that adverse effects of cross-talk between adjacent tracks are substantially eliminated. Nevertheless, the ability to accomplish high-density recording without deleterious cross talk accentuates another problem that was present all along but was overshadowed by the cross-talk problem, that is, the problem of drop-outs. These drop-outs occur at random and involve an absence of signal at a particular instance during reproduction typically related to mistracking caused by tape/head misalignment or by a stretched magnetic tape, or by particles of dirt or foreign material on the head or on the magnetic tape. These causes lead to the randomness of the drop-outs. Drop-outs adversely affect the audio as well as the video, and are manifested by a brief absence of sound in the audio signal and by a white line in the visual display in the case of the video signal. The ever increasing ability to raise the information density on the magnetic tape has made the random drop-out problem all the more important and one that requires a realistic and practical solution.